This invention relates to transducers, and more particularly to a composite transducer for sonar applications which has separate, non-interfering transmit and receive transducers.
The longitudinal vibrator-type transducer 10 of FIG. 1 is widely used in the prior art as a transmitter and receiver in sonar applications. The transducer consists essentially of an electromechanically active element 11 (typically a piezoelectric ceramic), a head mass 12, a rear mass 13, a bias rod 14, a pressure release system 15 and a waterproof housing 16, as shown in FIG. 1. The bias rod 14 provides a bias compressive stress on both the active element 11 and the pressure release system 15. Acoustic decoupling of the assembly of these components and the housing 16 is provided by the pressure release system 15. There are many variations of the transducer drawn in FIG. 1, but transducers of this general type have two characteristic frequencies that adversely affect receiving response. The two frequencies are the head and tail mount resonances.
Because of the phase shifts associated with resonances and the deterioration of a beam produced by an array of transducers as a consequence of phase shift differences between the transducers, a relatively flat receiving response is desired over a wide bandwidth. However, a typical transducer receiving response has uncontrolled head and tail mount resonances 20 and 21, respectively, as shown in FIG. 2. FIG. 2 shows a plot of receiving sensitivity versus normalized frequency n=f/f.sub.r, where f.sub.r is the open-circuit (constant-current) resonant frequency 21. The peak 22 in the response below resonance is due to the head mass-tie rod resonance. Similarly, the response minimum 23 is caused by the resonance of the spring-mass formed by the pressure release pad 15 and the rear mass 13.
In order to achieve a uniform or flat receiving response, the head and tail mount resonance frequencies must be equal, as well as the amplitudes of their resonances. Because of the difficulty of obtaining this balance in high volume production, damping is generally employed to compensate for any unbalance. The damping is often obtained by rubber bumpers 170 that are attached to the rear mass 13 and make frictional contact to the housing 16. A closely balanced transducer requires tight tolerances on both the material parameters and physical dimensions of the transducer. This adds significantly to the cost of the transducer, particularly in high volume production.
In addition to uniformity of receiving sensitivity, transducer self-noise is an extremely important performance parameter. Noisy transducers in a sonar array can cause a degradation of sonar system performance, as well as reveal the presence of the sonar platform. Longitudinal vibrator-type transducers such as that of FIG. 1, in particular have been found to generate extraneous noise when exposed to a changing hydrostatic pressure head. Typically, the extraneous noise is determined by measuring the open-circuit transducer voltage developed during pressure cycling. Polished contacting surfaces of the head mass 12, ceramic 11 and rear mass 13, very close tolerances on machine parts, and well-controlled alignment procedures have been found to be necessary to produce quiet transducers of the longitudinal vibrator type. These noise-quieting features have also added significantly to the cost of the transducer.
Also known in the prior art is a piezoelectric polymer which has low mass density and is mechanically flexible. These properties make the polymer more shock resistant than the prior art piezoceramics. Additionally, the characteristic impedance of the polymer more nearly matches that of water. Piezoelectric polymer film is presently made of polyvinylidene fluoride and is often referred to as PVF.sub.2. A polarization procedure must be used to render the polymer usefully piezoelectric. In one method of polarizing, both surfaces of the film are metallized to provide electrodes and a high d-c voltage is applied to the electrodes and held for about one hour at 100.degree. C. Subsequent cooling to room temperature under the applied field results in permanent polarization with the strongest piezoelectric effect in the direction transverse to the metallized surfaces of the film.
The polymer PVF.sub.2 has been used previously as a transducer for the transmission and reception of ultrasound signals. Since the acoustic power which may be transmitted with this material is limited, its use has been confined to low power applications such as in medical ultrasound.